This invention relates to a marker for locating a buried object, such as a gas pipe or water pipe, wherein the marker is buried near the buried object with the marker being capable of being detected thereby serving to locate the buried object.
An example of a conventional detection system using a marker for locating a buried object is shown in FIG. 1. According to this drawing, a detectable marker 2 is installed in proximity to a buried object 1. In this case, the buried object 1 is a metal tubular member, and the marker 2 is placed under the ground about 10 cm distant upward of the buried object 1. The marker 2, as shown in greater detail in FIGS. 2-4, comprises an antenna 6 of the passive resonance type which includes a coil 5 and a capacitor 4 sealed in a marker case 3 which is usually made of polyethylene or the like. In the conventional detection systems, when an electromagnetic wave of the resonant frequency of the marker is transmitted from above the ground toward the buried marker 2 by use of an apparatus comprising a locator 7 and a probe 8, the marker 2 responds and resonates with the electromagnetic wave which is detected by the locator 7 and probe 8. The best response is obtained when the marker 2 is in the horizontal position. Detection of the marker 2 in this manner serves to locate the buried object 1 with which it is associated. An example of such a detection system is found in an electronic marker system (EMS) available from Minnesota Mining and Manufacturing Company, St. Paul, Minn., U.S.A.
In such prior art detection systems, where the marker 2 is installed in proximity to the buried object 1, work requiring the excavation of the buried object 1, other excavation work or such natural phenomenon as an earthquake or sedimentation may change the position of the marker 2 from its original position of installation wherein the marker 2 is then inclined from the horizontal and about the axis 9 of the buried object 1 or displaced relative to the buried object 1. If an attempt is made to locate the buried object 1 by detecting the marker when it is thus inclined or displaced, an excavation might be made on the basis of detection made of the marker, with the result that the buried object is not reached directly causing a waste of labor and possible damage to the buried object. In situations where a wide variety of buried objects are installed in close distance from each other, there is a greater likelihood of inconveniences being caused by an excavation made on the basis of the detection made of an inclined or displaced marker.